Unlike the Gregorian calendar, the Julian calendar defines a leap year as a year that is evenly divisible by four with no exceptions. Therefore, the calendar is inaccurate by one day every 128 years. A common year has 365 days and a leap year has 366 days.

Like the Gregorian calendar, the Julian calendar has 12 months with 28 to 31 days each: January (31 days), February (28 or 29 days), March (31 days), April (30 days), May (31 days), June (30 days), July (31 days), August (31 days), September (30 days), October (31 days), November (30 days), and December (31 days). February has 29 days during leap years and 28 during common years.

The date January 1, 2001 A.D. in the Gregorian calendar is equivalent to the 19th day of December in the year 2000 A.D. in the Julian calendar.

Currently, the JulianCalendar is not used by any of the cultures supported by the CultureInfo class. Therefore, the JulianCalendar class can be used only to calculate dates in the Julian calendar.